This invention relates to cementing casing in a well bore. In particular, this invention relates to methods comprising reverse-circulating compressible cement compositions into single and/or multiple casing strings.
It is common in the oil and gas industry to cement casing in well bores. To do this, oftentimes a well bore is drilled and a casing string is inserted into the well bore. For reverse-circulation cementing, drilling mud and/or a circulation fluid then is circulated down through the well bore-by-casing annulus and then back up through the casing inner diameter to flush excess debris from the well. As used herein, the term “circulation fluid” includes all well bore fluids typically found in a well bore prior to cementing a casing in the well bore. After debris has been flushed from the well, a cement composition (typically comprising a hydraulic cement and a base fluid) may be placed into the well bore-by-casing annulus, and permitted to set therein.
Conventional methods of placing the cement composition in the annulus commonly involve pumping a cement composition slurry down the casing inner diameter, out through a casing shoe and/or circulation valve at the bottom of the casing, and up through the annulus to its desired location. Such methods often are referred to as conventional-circulation direction methods. Though conventional-circulation direction methods are the methods most commonly used for pumping cement compositions into well bores, these methods may be problematic in certain circumstances. For instance, a well bore may comprise one or more weak formations therein that may be unable to withstand the pressure commonly associated with conventional-circulation direction cementing operations. In these, and other circumstances, a second method of cementing may be employed wherein a cement composition slurry is pumped directly down the annulus, and permitted to set therein. This method often is referred to as a reverse-circulation direction method. When the cement composition slurry is reverse-circulated into the annulus, it displaces well fluids present in the annulus (e.g., the drilling mud and/or the circulation fluid) by pushing them through the casing shoe and up into the casing inner diameter. Reverse circulating the cement composition into the annulus may be useful in a variety of circumstances. For example, when the well bore comprises one or more weak formations, reverse-circulation cementing can reduce the pressure exerted against these weak formations during placement. A reduction in pressure can help prevent loss of whole cement fluid to the formation, which if it occurs can result in incomplete annular fill of the annulus with cement, need for extra cement being used, and/or require remedial operations to fill the annulus. The reduction in pressure by the reverse-circulation method is typically achieved by reducing the total pressure exerted on formations by the fluid being circulated out of the hole. Conventional-circulation cementing requires lifting a typically higher density cement up the annulus, with a total pressure exerted on the formations equal to the fluid density above them plus the friction pressure that occurs by circulating the cement up a typically narrow annulus. The total pressure exerted on the formation during reverse circulation is the fluid in the casing, which is typically a lower density well drilling fluid or other lighter weight fluid, plus the friction of pumping this fluid up a typically large diameter casing. The net reduction in total pressure exerted on the formation for reverse-circulation in comparison to conventional circulation can be significant and have a significant impact on reducing the potential for lost circulation, and the associated problems, from occurring.
Conventional reverse-circulation cementing operations may be problematic, however, for a variety of reasons. For example, it may be difficult to determine when the reverse-circulated cement composition has traveled through the entirety of the annulus and arrived at the casing shoe and/or circulation valve. Conventional attempts to solve this problem commonly have involved, e.g., disposing wireline tools within the casing string and including radioactive tracers in the cement composition to be reverse-circulated. For example, a wireline tool may be disposed within the well bore to sense the arrival at the casing shoe of the radioactive tracers within the cement composition.
Other conventional attempts to identify the arrival at the casing shoe and/or circulation valve of the reverse-circulated cement composition have involved, e.g., determining the annular volume to be filled with cement composition, and closely monitoring the volume of the cement composition reverse circulated into the annulus, to ensure that excessive cement composition is not pumped into the annulus. However, it may be difficult to accurately determine the actual volume of the annulus, which creates a risk of either a cement shortfall (e.g., pumping a volume of cement composition that is inadequate to fill the entirety of the annulus), or an excess of cement composition beyond that required to fill the annulus, which may result in the overflow of excess cement composition into the inner diameter of the casing string. Such inadvertent overflow of cement composition from the annulus into the inner diameter of the casing string generally is undesirable, because, upon the setting of such cement composition, the hardened cement typically must be drilled out of the casing string before further operations can be conducted in the well bore and/or the well can be put into service. Drilling out the set cement within the casing string requires extra labor and additional (often costly) rig time.
Furthermore, even when the annular volume is accurately measured, and the proper amount of cement composition is reverse-circulated, a portion of the cement composition still may inadvertently overflow into the inner diameter of the casing string. This phenomenon may be referred to in the art as “U-tubing.” The risk of a portion of the reverse-circulated cement composition inadvertently U-tubing into the inner diameter of the casing string may be exacerbated by the fact that the equipment commonly used to prevent U-tubing (e.g., float collars) may not be feasible for use in reverse-circulation operations. Accordingly, conventional attempts to prevent U-tubing during reverse-circulation operations commonly have involved pressurizing the casing string inner diameter to prevent, or impair, the entry therein of the reverse-circulated cement composition. However, these methods have been problematic because they undesirably may create a micro-annulus between the casing string and the set cement sheath that is formed by the setting of the reverse-circulated cement composition in the annulus. This micro-annulus may be undesirable because it may complicate bond-logging efforts (operations which evaluate the quality of the cement job and determine whether the annulus has been sealed properly) and/or allow for a flow path of fluids and/or gases in the annulus which one is attempting to seal with the cement.